The "dorsal organ" in Crustacea Malacostraca. 69 



surface of the head near the posterior margin in the Isopod 

 Bathynomus giganteus; theorgan which shall be mentioned later 

 on was briefly described, figured and compared with the organ 

 discovered by Caiman (1896) in Anaspides tasmanice; it was 

 added, that I had found a vestige of the organ in the allied form 

 Cirolana borealis. -- The statements in the literature on the 

 organ in larvae of Sergestes arcticus are quoted above. 



Let us sum up. The median dorsal organ has been found 

 in embryos of representatives of the following orders of Mala- 

 costraca: Nebaliacea, Mysidacea, Cumacea, Tanaidacea, Isopoda, 

 Amphipoda and perhaps some Decapoda. The Embryologists 

 tell us that it, at least generally, is of glandular nature and dis- 

 appears early; on its function nothing is known, besides it is 

 unknown in the embryo of Stomatopoda. In adult forms it 

 has been found in Anaspides of the order Anaspidacea; in Ba- 

 thynomus and a species of Cirolana of the order Isopoda; finally, 

 according to the quotation from Giesbrecht, in young and adult 

 forms of the order Euphausiacea. In larval stages it is only 

 known in Sergestes arcticus. 



III. The "dorsal organ" in adult Malacostraca 

 and some larvae. 



It has not been my intention to inspect every genus avail- 

 able, but types of families or genera have been selected and 

 examined with a good pocket-lens ; it was found that within the 

 same family, sometimes even within the same genus, an organ 

 could not infrequently be discovered in a large species, but not 

 with any certainty in small forms. The following enumeration 

 shows that the organ has a much wider occurrence than might 

 have been supposed. 



i. Nebaliacea. 



Nebalia bipes O. Fabr. In turning the animal a little to 

 and fro so that the light changes on the smooth, shining dorsal 



